1. Field
The following description relates to a tracking method on an optical disc, and more particularly, to a single beam tracking method, an optical pickup device, and an optical disc drive device adopting the method.
2. Description of the Related Art
Optical pickup devices for an optical disc drive device have a structure in which a plurality of optical systems for maintaining multimedia compatibility are integrated into one body. For example, optical pickup devices having compatibility with a compact disc (CD), a digital versatile disc (DVD), and a Blu-ray disc (BD) include an optical system for a CD/DVD and an optical system for a BD separately. Optical pickup devices generally include an optical system for a CD/DVD and an optical system for a BD which share some of the components needed for reading and/or recording to and from the respective media. For example, the optical path of optical pickup devices used for BD media generally overlap with at least part of the optical path of an optical pickup device used for CD/DVD media. Such optical pickup devices have a design in which optical elements are very densely disposed. Thus, such optical pickup devices have a very low degree of design freedom.
An optical system may be designed having a plurality of lenses, such as a lens for a CD/DVD and a lens for a BD. The plurality of lens are arranged on a tracking control line in a tracking control direction (i.e., in a so-called radial direction) such that a line passing through the plurality of lenses passes through a rotational center of a medium (or turntable). This arrangement is generally applied to an optical pickup device for multimedia. However, the plurality of lenses need to be arranged in a direction perpendicular to the tracking control direction (i.e., in a so-called tangential direction) according to design conditions. In this regard, the lens for a CD/DVD or the lens for a BD deviates from the tracking control line. For example, when the lens for a CD/DVD is designed to be located on the tracking control line in the radial direction, the lens for a BD is located in a position that deviates from the tracking control line. In other words, the lens for a BD is located in an offset position. Thus, it is not easy to perform tracking control on the optical system in which an objective lens is located in the offset position, by using a so-called 3-beam method.